City News

New energy-efficient RO membranes installed at water plant

Post Date:02/15/2018 2:31 PM

Drinking water supplied by the City comes from two sources: Lake Washington and the Floridan Aquifer. The lake water is treated using the Actiflo process, while groundwater from the aquifer is treated through reverse osmosis (RO). This technology is used to remove a large majority of contaminants from water by pushing the water under pressure through semipermeable membranes. The water that makes it through the RO membrane is called “permeate” or “product water.” The RO treatment process includes two treatment trains, each designed to produce 2.5 million gallons per day of permeate. There are 1,008 membrane elements within the pressure vessels.

The City’s RO plant has been in operation since 1995. The original RO membranes were first replaced in 2005 and for the second time were replaced earlier this month. The typical service life of the membranes is five to seven years, so the ones just replaced exceeded expectations.

The new membranes are energy-efficient hybrids, which will save money over the life of the membranes and may provide higher water yield. A membrane pilot study was conducted in 2017, and the newer low-pressure membranes were selected. “The study provided options for the use of the newer low-pressure membranes that require less energy consumption than the existing membranes,” said Utilities Engineer Jennifer Spagnoli. “We are assuming from the pilot study that they may provide $250,000 in energy savings per year. In order to receive the energy savings, the soft starts for the membrane feed pump motors were replaced last year with variable frequency drives (VFDs).”

Water Production Superintendent Fred Davis noted that while the efficiency of the new membranes has increased over time, the costs have gone down. “In the ‘90s, it would have cost more than one million dollars to replace them.” This current project is costing less than half that at a price of $488,608. H2O Innovations of Vista, California is the contractor for the project. To keep the RO plant off-line for as short a period as possible, the replacements were fast-tracked and completed in one week.